I have an iPhone 5S with iOS 8.4.
I recorded a video using the built-in Camera app. This is how the built-in Photos app shows the video:
Then I opened up the iMovie app (version 2.1.2), created a new project, imported that video, did not do any editing, and then exported the video using the "Save Video" option (exported as 1080p). When I went into the built-in Photos app, this is how it showed the exported video:
Why has the exported video been shrunk, and had part of the top and bottom removed? Instead of being a tall rectangular video like the original was, the exported video is a square with a lot of black space around it.